Intercalation and Pattern Regulation in Hydroids

Several topics and concepts in developmental biology, such as induction of secondary axes (as in amphibians), the rules of distal transformation (valid, e.g., for insect and vertebrate limbs), and the rule one piece regenerates, the other one duplicates (valid also for imaginal discs) originally emerged from pioneering studies in hydroids. In addition, one finds phenomena of intercalation, i.e., the insertion of intervening parts when tissues of distant origin are confronted with each other. However, since not only short-range but also long-range interactions contribute to pattern regulation, one observes more regulative flexibility as compared to insect systems.     

Guanylate Cyclase Activity in Dictyostelium discoideum and Its Increase during Cell Development

Following consumption of the food supply, cells of the cellular slime mould Dictyostelium discoideum aggregate and form a multicellular organism. The mechanism for cell aggregation is chemotaxis. The chemotactic signal in D. discoideum is released periodically from aggregation centers and propagated from cell to cell. cAMP mediates cell aggregation by acting as chemotactic attractant and as propagator of the signal. cAMP signals are measured by cell-surface receptors. Recent evidence indicates a role for cGMP during cAMP-mediated cell aggregation in D. discoideum .
During cell differentiation to aggregation competence, cAMP binding sites appear at the cell surface, and the activity of the enzymes adenylate cyclase and phosphodiesterase increases several-fold. In the present work we investigate the synthesis of cGMP in D. discoideum . Conditions for the assay of guanylate cyclase in cell homogenates are described. Guanylate cyclase activity was followed during cell differentiation to aggregation competence and found to increase fourfold. These results indicate that cGMP is involved in cell differentiation of D. discoideum . In contrast to adenylate cyclase, which is activated by cAMP, guanylate cyclase was under our conditions activated neither by cAMP, nor by folic acid.     

Tumulduria incomperta and the case for Tommotian trilobites

The enigmatic fossil Tumulduria incomperta Missanhevsky 1969 from the basal Tommotian Stage at 'Dvortsy' on the Aldan River, Siberian Platform, is reinvestigated in light of the report (Fedorov et al. 1979, Dokl. AN SSSR 249) of trilobite remains from the same beds. Comparisons of these supposed trilobites with old and new collections from 'Dvortsy' leave no doubt that they are identical to Tumulduria incomperta. Tumulduria is represented by phosphatic plates with a crude bilateral symmetry, consisting of a central longitudinal rounded ridge flanked by flat lateral portions. They are built of growth lamellae overlapping each other along the axis of symmetry. The surface carries prominent transverse folds and lamellar terminations. There is considerable morphological variation, and the similarity of some specimens to trilobites is only superficial. Tumulduria is interpreted as a bilaterally symmetrical metazoan with a dorsal protective plate; it probably represents a short-lived group that left no descendants.     

Within-lake patterns in depth penetration of emergent vegetation

SUMMARY. 1. Within-lake relations of wave exposure (WE), and substratum softness (cone penetration depth; CPD) and organic content (loss on ignition; LOI), to water depth penetration of the emergent vegetation (DPE) was investigated in seven eutrophic lakes in southern Sweden, ranging in area from 1 to 46km².
2. There was a positive relationship between WE and DPE within lakes. This relationship, however, only occurred for sites with relatively soft substrata, for which CPD and LOI were negatively related to both WE and DPE.
3. Analysis of aerial photographs revealed that expansion of the emergent vegetation towards open water, or recession from open water, was not related to wave exposure or water depth, except in one lake where expansion mainly occurred at high exposures.
4. For relatively static vegetation on soft substrata, regressions with CPD°.⁵ explained 62–88% of the within-lake variation of DPE. These regressions did not differ among lakes. Expanding and recessing vegetation were significantly dislocated towards shallower and deeper water, respectively, than predicted from the regression models for static vegetation.
5. Phragmites australis dominated at the lakeward edge of the emergent vegetation, followed in frequency by Typha angustifolia . More broad-leaved species were generally restricted to shallow water and very soft substrata.
6. The results suggest that changes in the distribution of emergent vegetation in moderately wave exposed eutrophic lakes can be predicted largely from substratum character and water depth.     

Influence of submergence on growth of seedlings of Scirpus lacustris and Phragmites australis

• 1 Seeds of Scirpus lacustris and Phragmites australis were germinated in early June, and twenty-four seedlings of each species were subsequently exposed to submerged conditions (eight seedlings at each of the water depths 0.2, 0.4 and 0.8m), in outdoor 500–1 tanks in southern Sweden. Weight and shoot length of the plants were measured in September.
• 2 The Phragmites seedlings did not show any significant growth when submerged. The Scirpus seedlings, however, developed submerged leaves and exhibited considerable submerged growth. One Scirpus plant, in shallow water (0.2m), had developed an aerial shoot by September. Shoot length of the remaining (submerged) Scirpus plants was positively related to plant weight within water depth treatments, and was higher, in relation to plant weight, in deeper water. Mean weight in September of the submerged Scirpus plants decreased with increased water depth.
• 3 In south Swedish lakes with a lowered water table, Scirpus often occupies large areas on the lakeward side of the reed belt, which is generally dominated by Phragmites. The differences between the two species, in performance of submerged seedlings, suggest that this zonation may be created through successful submerged seedling establishment of Scirpus on the lakeward side of Phragmites.